The invention relates to a slip-controlled brake system with a power-assisted, brake pedal-actuated master cylinder, with sensors for the determination of the behavior of rotation of the wheels and of the vehicle speed or of analogous measurands. Valves are provided in the hydraulic fluid supply lines going from the master cylinder to the wheel cylinders and an electronic switching arrangement is provided for the logical linkage and processing of the sensor signals and for the generation of valve control signals. The power-assisted pedal force directed on the master cylinder is temporarily compensatable in part or in full by an external power acting in opposite direction. The wheel cylinders are connected, during or within the phase of reduced pressure in the master cylinder, to the master cylinder through the hydraulic fluid supply lines or are cut off due to an actuation of the valves depending on the instantaneous behavior of rotation of the relevant wheel or of a group of wheels. A servo unit in accordance with the principle of a vacuum brake power booster is provided for the generation of the auxiliary power and of the opposedly directed external power. The servo unit comprises a vacuum cylinder with a power piston which, in accordance with the pressure differential in the power chambers on either side of the power piston, transmits a pressure to a push rod. The push rod acts upon the master-cylinder piston. The pressure differential in the two power chambers is controllable with valves depending on the brake pedal effort.
A slip-controlled brake system of this type is described on German patent application P No. 33 17 629.9 which corresponds to copending U.S. application Ser. No. 609,573 filed 5-11-84 in which the piston rod coupled to the brake pedal acts, either directly on through a column of hydraulic fluid, on the push rod sliding the master cylinder piston. The push rod is coupled to the piston of the vacuum cylinder. This arrangement has the disadvantage that the master cylinder piston may move after the closure of the solenoid valves inserted in the brake lines thereby reaspirating hydraulic fluid from the supply tank of the master cylinder through the primary cup of the master cylinder piston. It is a further disadvantage of the prior art brake system that a reduction of the pressure in the brake lines takes place exclusively when the piston rod carried out a stroke in contrast to the direction of pedal operation. The consequence is that the driver cannot develop any feeling for the degree of brake power applied.
The object of the present invention is to create a slip-controlled brake system in which the servo unit provided for the generation of the auxiliary power and of the opposedly directed external power and operating according to the principle of a vacuum brake power booster is arranged such that it gives the driver a precise feeling for the brake power applied at each moment. In this context, the degree of the pedal effort applied by the driver should be independent of the time sequence of the control cycle, and the reduction of the pressure in the brake lines should take place with particular quickness in the presence of a comparatively moderate vacuum with the sections of the tubing and hoses necessary for the brake system being of a comparatively small size. Beyond this, the brake system should offer particular reliability in operation, and it should provide a simple set-up and low manufacturing cost.